feat(postgres): add locking attribute to #[sqlx::test]

[mpecan]

Adds locking = false support to #[sqlx::test] to skip advisory locks during test database setup and migrations. This is the #[sqlx::test] analog of Migrator::set_locking(false) (#2063).

Since #3753 switched test setup to pg_advisory_xact_lock, databases that speak the PostgreSQL wire protocol but don't implement advisory locks (e.g. CockroachDB) cannot use #[sqlx::test] at all.

Usage:

#[sqlx::test(locking = false)]
async fn my_test(pool: PgPool) -> sqlx::Result<()> {
    // ...
    Ok(())
}

Locking remains enabled by default. Disabling it means concurrent test runs may race during schema setup, which is acceptable for databases that handle schema DDL differently.

Does your PR solve an issue?

Partially addresses #198 (CockroachDB support).

Is this a breaking change?

No. Default behavior is unchanged (locking = true).